The present invention relates to a screen plate used in paper and pulp industry to separate foreign matter such as shives and undefibred paper fragments, plastic fragments, hairs, sand, metal fragments and the like from pulp slurry.
In paper and pulp industry, screens have been used to separate foreign matter from pulp slurry at a consistency of the order of 0.3-2.5%.
Particularly screens having screen plates with slots are widely used, because the slots may be narrowed and the fine slots with the narrowed width are very effective for separating adhesive foreign matter such as hot melt which tend to become very fine spherical particles.
FIGS. 4 and 5 show a screen plate having slots.
FIG. 4 is a plan view while FIG. 5 is a sectional view. Reference numeral 1 represents a screen plate; 2, slots; 3, a scraper blade; and 7, land portions. Width of the slots 2 is narrow at the pulp slurry inlets and is wide at pulp slurry outlets so that the slots are prevented from being clogged.
The narrow width of the slots 2 for enhancement of screening efficiency will cause a problem that a quantity of the slurry passing through a unit area of the screen plate is naturally decreased and consequently screening capability is lowered. Meanwhile, there is a demand for passing high consistency pulp slurry through a screen plate for facilitation of simplifying a pulp slurry thickening device as well as for attainment of reducing power required for transporting the pulp slurry. Such high consistency pulp slurry is difficult to pass through a screen and a reject quantity must be increased so that a considerable quantity of good-quality fibers are discharged as reject.
As shown in FIG. 5, the scraper blade 3 may be of the type which sucks foreign matter hydrodynamically by negative pressure induced along rear edge portions of the blade 3. Alternatively, a blade 3' as shown in FIG. 6 and of a mechanical type may be used which scrapes the surface of the screen.
The screen plates 1 are in the form of a cylinder or a disk.
In order that the high consistency pulp slurry may pass through the screen plate of the type described above and in increased quantity, frequency of the scraper blade passing for scraping the openings must be increased. To this end, it suffices to increase a scraping blade speed or to increase the number of scraping blades; however, the former is disadvantageous in that the power for driving the scraping blade must be increased and the latter results in hindrance of the pulp slurry passage.
In order to overcome the above-described problems, various proposals for changing the shape of the screen plate itself to increase the quantity of pulp slurry passing through the screen plate have been made and attained some improved effects. For instance, (1) there has been proposed a screen plate (Japanese Patent 2nd Publication No. 36958/1960) in which, as shown in plan view in FIG. 7 and in section in FIG. 8, an elongated groove is formed on the land portion 7 between the adjacent slots 2 so that the flow of pulp slurry at inlet side of the screen plate 1 is disturbed and consequently the pulp fibers are oriented in the direction of the pulp flow in the slots 2; (2) there has been also proposed a screen plate (Japanese Patent 2nd Publication No. 25138/1978) in which, as shown in cross section in FIG. 9, the downstream side edge of each slot 2 is lower than the upstream side edge thereof in the direction of the flow of pulp slurry so that leading ends of the pulp fibers flowing over in parallel with the screen plate may easily enter the slots 2, thereby increasing the quantity of pulp slurry passing through the screen plate; and (3) there has been further proposed a screen plate (as disclosed in Japanese Patent 1st Publication No. 137594/1984) in which, as shown in section in FIG. 10, each slot 2 is formed through the bottom of each groove c so that leading ends of pulp fibers flowing over in parallel with the screen plate may easily enter the slots 2, thereby increasing the quantity of pulp slurry passing through the screen plates.
However, in the case of the screen plate described in (1), screening capability is not always increased to a satisfactory extent. In the cases of the screen plates described above in (2) and (3), turbulence occurs at each of the slots 2 so that the openings 2 tend to be worn and increase in size, causing the decrease in screening efficiency. Furthermore, from the standpoint of the structure of these screen plates, the depth of groove is limitative (and in general is of the order of 0.5-0.7 mm) so that the side wall b on the downstream side is worn rapidly and as a result, screening capability is rapidly decreased and service life of the screen plate is considerably short.
In the above-described conventional structures, the screen plates are 3-8 mm in thickness t; the width d of the slots is of the order of 0.1-1 mm; and the pitch e of the slots is of the order of 4-15 mm.
The present invention was made to substantially overcome the above and other problems encountered in the conventional screen plates and has for its object to provide a screen plate which can increase the quantity of pulp slurry passing through the screen plate while maintaining a high degree of screening efficiency and which ensures a long service life of the slots screen plate.